Nuclear power plant steam generators must be able to withstand loading forces, such as that occurring under seismic events and postulated accident conditions, without damage to either the shell or the internals of the steam generator, such as the tube bundle and associated supports. The steam generator must also be restrained from large movements during these events so as to prevent damage to the associated piping. Resisting these loading forces requires an assembly which can accommodate steam generator movement since the disposition of the steam generator varies during heat up and cool down of the reactor coolant due to thermal expansion of both the piping attached to the steam generator as well as the generator shell.
From a cool down temperature which is typically about 70.degree. F. (21.degree. C.) to a typical operating temperature of about 550.degree. F. (288.degree. C.), the steam generator will move laterally due to thermal growth of the attached piping and grow or thermally expand in the vertical and radial directions due to thermal growth of the steam generator shell. The direction of lateral growth is along a line corresponding to the hot leg from the reactor vessel which enters the steam generator (see FIG. 1). After heat up of the steam generator, the lateral supports radially spaced about the upper portion of the steam generator and secured to a typically concrete housing surrounding the steam generator, which is a component of the containment building, must not leave a gap greater than 1/16 in. (0.16 cm). This nominal gap, which is essentially zero, is the maximum allowable to insure adequate support under typical loading forces.
As shown by FIG. 1, the means typically employed for resisting these loading forces is the use of snubbers disposed between the steam generator support, commonly referred to as a ring girder, and the concrete housing. Snubbers are typically mechanical or hydraulic devices, much like an automobile shock absorber, which allow for very slow (e.g., 0.05 in./min.) extension and retraction such as occurs during reactor coolant loop heat up and thermal expansion of a steam generator, but become rigid and resist sudden displacement such as would occur under seismic loading. These snubbers are typically disposed on the steam generator in the direction of lateral growth, and oriented generally parallel with that line. Snubbers are relatively large and can require extensive maintenance and testing to insure their proper operation. The use and care of numerous snubbers can thus result in considerable expense to a nuclear power plant operator. Radially spaced about the steam generator in areas not supported by the snubbers are vertically-faced support blocks (or bumpers) which also provide restraint for the steam generator during these loading events.
It is noted that relatively maintenance-free bumpers have been used for steam generator restraint wherever the gap between the bumper and the steam generator decreases during plant heat up (i.e., in the direction of lateral movement), such that a nominal zero inch gap can be achieved during the plant's normal operating (i.e., hot) mode. (See FIG. 2). However, this design does not provide for a 0" nominal gap at bumper locations where the gap typically increases during plant heatup to its normal operating condition, and requires shims between the bumper and the housing.
Although conventional steam generator upper support arrangements can adequately resist these loading forces, the size and number of snubbers needed can be quite expensive to use and maintain. It would thus be advantageous if the need for snubbers could either be totally obviated, or at least their number reduced. Such a support assembly must also be able to fully support the steam generator after heat up so as to allow for thermal expansion of the shell of the steam generator.